The present invention relates to a distributed feedback semiconductor laser (hereinafter referred to simply as a "DFB laser") into which a photodiode having a monitor function is integrated.
Since a DFB laser has incorporated therein a diffraction grating of excellent wavelength selectivity, it is capable of achieveing a single wavelength operation, and is now being developed as a light source for low-loss optical fiber communication and the like. In case of employing a semiconductor laser, it is general practice to monitor the magnitude of its output with a photodiode for the purpose of keeping the optical output constant. In this instance, the semiconductor laser and the photodiode are usually formed by different devices independent of each other. Of semiconductor lasers, the DFB laser not only enables the single wavelength operation but also saves the necessity of a reflecting end face which is requisite for conventional semiconductor lasers, such as a cleavage plane; so that it is suitable for monolithic integration therewith of an element equipped with a function other than laser oscillation. In particular, the above-mentioned photodiode for monitoring the magnitude of the optical output can easily be integrated with the DFB laser.
However, the conventional DFB laser has a defect that the monitor sensitivity undergoes substantial variations with temperature fluctuations.